Production of replacement teeth from a three-dimensionally determined and digitized positive model

ABSTRACT

A method for the production of replacement teeth ( 5 ) from a three-dimensionally determined and digitized model, whereby for improved handling, cost and quality of the produced replacement tooth ( 5 ) a method including the following steps is carried out: inputting the data for the digitized model, treating the inner surface ( 9 ) of the surface model with a given offset value to generate a cement gap ( 7, 8 ) between the tooth replacement ( 5 ) to be made and tooth stump ( 1, 2 ) and calculation of a program for machining the blank or a mold by means of a machine tool.

BACKGROUND OF THE INVENTION

The invention relates to a method for the production of a dentalprosthesis to be fastened to a tooth stump consisting of athree-dimensionally mapped and digitized positive model which comprisesthe following steps:

-   -   inputting the data of the digitized positive model;    -   machining the inner surface of the positive model with a preset        offset value to form a gap between a future dental prosthesis        and the tooth stump, and    -   calculating a program for machining a blank by means of a        machine tool to produce the dental prosthesis.

In particular, the invention relates to the field of producing basicstructures for dental prostheses, in particular for dental crowns and/orbridges for fastening to prepared natural and/or artificial tooth stumpsor the like.

A number of devices and methods for producing artificial dental bridgesand crowns are known. Generally, after the dental preparation in whichthe teeth used for anchoring are prepared by grinding for receiving acrown or bridge or for which, e.g. a pin is implanted, an impression ofthe tooth stump, the surrounding area and jaw is made. This is usuallydone with silicone sealing compounds, but other materials are alsoknown.

A so-called master model can be made from the impression by means of aplaster cast. This master model shows the situation in the patient'smouth positively. In this model, the dental technician with hishandicraft skills fashions a model of the basic structure of the dentalprosthesis from wax or from plastic which melts at a low temperature orhardens in a polymerizing manner (positive model). In this case, thedental technician can also take the counter occlusion of the other jawinto account by means of the master model in hand.

Traditionally, the model produced by the dental technician is embeddedand melted in heat-resistant substances. The basic structure can be madeby precision casting in conventional metal dental alloys in the moldthus produced.

For cosmetic reasons, a facing in ceramic or plastic is usually alsomade, at least in the area of the front teeth.

It is known from WO 99/47065 to completely digitalize the outer andinner surface of the structural model after a wax model has been formed.A model which inadequately reflects the situation in the patient's mouthis then mathematically completed with respect to the three-dimensionalouter and inner surface. The result of the digitalization and amathematical completion should represent a digital description of thecomplete surface of the basic structure of the prosthesis.

In order to be able to machine a blank consisting of porous ceramicswith the data obtained from the digitalization of the wax model, it isdescribed in detail in WO 99/47065 that the dimensions of the surfacemodel of the digitized body are linearly enlarged in all spatialdirections to compensate for the shrinkage of the blank duringsintering. The enlargement factor f should thereby result from aspecific function according to the ratio of the density of the blank anda basic bridge structure produced therefrom.

The control commands for a processing machine are to be generated fromthe data of the enlarged surface with which the enlarged basic bridgestructure is to be carved out of the blank. In comparison to themathematically enlarged surface of the basic bridge structure, no offsetis provided, whereby the machined surface is to obtain exactly the samemeasurements vis-á-vis the digitized body after the sintering shrinkage.Furthermore, no subsequent processing should take place.

In addition, it is proposed in WO 99/47065 to use a blank for theproduction of the basic bridge structure in which a machine or visuallyreadable identification code containing the enlargement factor f isaffixed to the blank itself or to its packaging, to a tag or an enclosedpackaging slip.

The wax model of the basic bridge structure should be producedpositively on a plastic impression of the negative form of the preparedtooth stump produced from a silicone mass, whereby the tooth stumpsshould first be coated by hand with a spacer coating composition tolater form a cement gap.

The digitalization is accomplished mechanically or optically. For thispurpose, reference is made to methods for the digitalization in themouth of a patient on a prepared tooth stump or to a model, said methodsbeing known, for example, from U.S. Pat. No. 4,182,312 with respect to amechanical digitalization and from EP 0 054 785 A1 with respect to anoptical digitalization.

The fundamental disadvantage of the mechanical digitalization known fromU.S. Pat. No. 4,182,312 is in the fixing of the mechanical scanningdevice to the patient since the scanning is to take place directly inthe oral cavity of the patient. The secure handling of the device in thenarrow oral cavity is equally problematic. A processing machine forproducing dental prostheses should be controlled directly with thescanning of teeth and surrounding tissue as in a duplicating millingmachine.

To this end, a probe having a transmission rod securely fixed to it mustbe moved by the dentist over the surfaces in the patient's mouth thatare of interest. A complete detection of the surface requires very manyscanning movements, which is very stressful for the patient due to thetime required. Furthermore, the probe tips must be changed depending onthe shape of the processing tool.

With the method described in EP 0 054 785 A1, an image recording head isto be inserted into a patient's mouth. This image recording head is todetect a three-dimensional image of a tooth cavity or the like. For thispurpose, the image data is to be shown on a computer screen, so that adentist can check to see whether the positioning of the image recordinghead enables a sufficiently accurate image. If necessary, the imagerecording head can be changed accordingly to a more favorable position.

When a proper position has been obtained, a three-dimensional image ofthe tooth cavity or the like should be formed spatially true tosize—without further explanation. The appropriate data is then to becompleted by interpolation and manual processing of the data set in themanner of a CAD construction, until a corresponding dental prosthesisbody has been completely formed. The corresponding data should then beused to make a suitable blank in order to produce a suitable dentalprosthesis directly from the image while avoiding the aforementionedskilled production steps.

The awkward manipulation with the camera in the patient's mouth was alsofound to be disadvantageous in practice with this method, in particular,it requires great discipline on the part of the patient.

Furthermore, as described in the aforementioned document, it isnecessary to coat the tooth to be mapped with a powder to obtain definedreflection conditions, since the natural dental material has translucentproperties. Due to the translucent properties, light could otherwisepenetrate partially uncontrolled into the tooth stump to be measured andperhaps be reflected in deeper layers which would result in aninaccurate result. However, the coating with a reflection powdersimultaneously increases the inaccuracy by the application of the powderwhich will inherently and, based on the restricted conditions in thepatient's mouth, always be irregular in practice. The limited resolvingpower of the image recorder and the difficult lighting conditions in themouth to be mapped are also disadvantageous.

A method for producing dental prosthesis parts is known from DE 196 42247 C1 according to which a prepared tooth is digitized in order to thenproduce a dental prosthesis taking the digitized model teeth intoconsideration. To produce a dental prosthesis, according to WO 94/27523,a tooth is measured, a part of a tooth is prepared, a tooth impressionor a copy of the tooth is made. The triangulation method is used for themeasuring.

A powder-metallurgical production process for an accurately shapeddental prosthesis is known from EP 0 774 933 B 1. In this case, thethree-dimensional optical or mechanical inputting of the prepared toothtakes place directly in the mouth or on a plaster model. The cementlayer, by means of which the dental prosthesis is joined with theprepared tooth, is also taken into consideration when producing thedental prosthesis.

Independent of the type of production of the dental prosthesis, foraesthetic reasons, care is taken to make it as slender as possible. Fromexperience, the dental model is also formed primarily from an aestheticpoint of view to have sufficient space for the subsequent facing. As aresult, the necessity for a sufficient mechanical construction isdisregarded and the life of the dental prosthesis produced in accordancewith it is adversely affected.

SUMMARY OF THE INVENTION

The object of the present invention is to further develop a method forthe production of dental prostheses of the aforementioned type in such away that the dental prosthesis of this type has sufficient strength toachieve a long service life, it being simultaneously ensured that thedental prosthesis has a sufficient aesthetic effect and corresponds tothe appearance of a natural tooth. An improved method with respect tohandling, cost efficiency and quality of the dental prosthesis thusproduced is also to be provided.

According to the invention, the object is essentially solved in that asmaterial for the dental prosthesis one is employed which has, in normaluse of the dental prosthesis, a maximum tensile stress occurring whichcorresponds to about 0.1–0.7 times the tensile strength of the materialemployed.

In particular, the maximum tensile stress in the dental prosthesisshould correspond to ≦0.5 times, in particular 0.2–0.5 times, thetensile strength of the material used.

By means of the teaching according to the invention, the dentalprosthesis is designed in its

dimensioning with respect to the loading due to tensile stresses,whether it be, for example, in the wall thickness, whether it be forexample in the material cross-section or the connection radii in bridgeelements in the area between supports formed by tooth stumps. As aresult, it is ensured that the dental prosthesis has the requiredstrength without having to accept the loss of its aestheticalappearance. In other words, a slender appearance can still be givenwithout the risk of failure occurring under normal stress of the dentalprosthesis. In its dimensioning, the dental prosthesis is therebydirected, with respect to the especially strongly stressed areas, to thestrength of the material used. For example, if Al₂O₃ is used as ceramicmaterial for a dental prosthesis, then the tensile strength of thematerial is 350 N/mm², so that a maximum tensile stress between 35 and245 N/mm², in particular about 175 N/mm², may occur in the area which ismost stronly loaded. In bridges, this maximum tensile stress occurs inthe transitional area between the bridge elements and the posts formedby the tooth stumps. If Y₂O₃—stabilized ZrO₂, which has a strength of650 N/mm², is used as ceramic material, then the dimensions of thedental prosthesis must be designed accordingly such that a maximumtensile stress of between 65 and 455 N/mm², in particular ≦325 N/mm²,occurs in the most strongly area.

Furthermore, the offset value should be preset dependent on the axis.

Furthermore, by means of the teaching according to the invention, areliable and precise gap for receiving cement or other binding agentsfor securing the dental prosthesis on the prepared tooth stump can beobtained with the desired extent of the clearances. In this way, therisk of the known methods resulting from the manual application ofspacer coating compositions with respect to maintaining the optimalthickness, uniformity and reproducibility of the application can beexcluded. In particular, due to the axis-dependent formation of the gap,it is ensured that an optimal fit of the dental prosthesis on the toothstump and their interconnection is made possible.

It is advantageous for all current fastening techniques for a crown or abridge on a tooth stump if the offset value is up to 150 μm.

The smallest gap dimensions in the area of the contact surface of adental prosthesis and tooth stump, and thus an especially goodresistance to a future caries attack, can be obtained if the gap differsfrom a low value at the periphery of the cavity defined by the innersurface of the positive model to a larger value in the tip of thecavity, in particular, if the gap on the periphery of the cavity is lessthan 5 μm, preferably less than 2.8 μm, especially preferred not morethan 1.5 μm.

The method of the invention is also especially secure without operatorintervention if it is further characterized by the following step:conducting a plausibility test of the data with reference to the data ofadjacent areas of the positive model and, optionally, issuance of awarning.

In a further advantageous embodiment, the method is furthercharacterized by the following step: replacement, preferably ofindividual missing data points or of non-plausible data points byinterpolated values with reference to the data of adjacent areas of thepositive model and smoothing the outer and/or inner surface of the dataof the positive model by means of a smoothing function.

To prevent a formation of hollow spaces between dental prosthesis andtooth stump, from which a further caries attack of the tooth stump andthus a destruction of the support tooth of a bridge can usually beexpected, the method is advantageously further characterized by thefollowing step: conducting a test of the data of the inner surface ofthe positive model for undercuts with reference to the data of adjacentareas of the positive model and, optionally, issuance of a warning.

For an especially high security against breakage of the dentalprosthesis during later use by the patient, the method is furthercharacterized by the following step: calculating the wall thickness ofthe positive model by comparing the data of the inner surface of thepositive model with the data of the outer surface of the positive modeland comparing the value of the wall thickness with a wall thicknessminimum value, correcting the data of the outer side of the positivemodel such that at least the wall thickness minimum value is maintainedand/or a warning issued, in particular if the minimum value of the wallthickness is at least about 0.5 mm, preferably about 0.1 mm, inparticular at least about 0.3 mm.

To ensure a favourable shaping of the dental prosthesis, it is alsoadvantageous if it is ensured by comparison of the radii of the outerand/or inner surfaces of the positive model with a minium radius of atleast about 0.1 mm, preferably about 0.2 mm, in particular 0.3 mm, andcorrection of the data of the outer and/or inner surface of the positivemodel, that at least the minimum radius is obtained and/or a warningissued.

To produce an especially stable and durable bridge, the method isfurther characterized by the following step: calculation of the materialcross-section in bridge elements in the area between two posts formed bytooth stumps, comparison of the cross-section with a minimum value,correction of the data of the outer side of the positive model such thatat least the minimum value is maintained and/or issuance of a warning,in particular if the minimum value is at least about 2 mm², preferablyabout 5 mm², in particular at least about 7 mm².

To prevent a too narrow cavity of the dental prosthesis when preparingthe top of the tooth stump which does not have a sufficient roundnessfor conventional milling cutters, the method is further characterized bythe following step: calculation of the radii of the inner surface of thepositive model, comparison of the values of the radii with a minimumradius, correction of the data of the inner surface of the positivemodel such that the cavity is enlarged to such an extent that at leastthe minimum radius is obtained and/or a warning issued.

Depending on how the machine tool to be used is controlled, it can beexpedient if the method is further characterized by the following step:machining of the positive model with a preset offset value to adapt itto the tool contour during subsequent processing.

To produce dental prostheses from blanks which are subject to a changein dimensions after the mechanical processing during production, e.g.ceramic blanks that are not sintered or presintered, which must then becompletely sintered after the shaping work and thereby shrink, themethod according to the invention is especially advantageously furthercharacterized by the following step: processing the data of the positivemodel with an enlargement factor to compensate for shrinkage or swellingof the material for a dental prosthesis during production, in particularfor a good fit when the enlargement factor is non-linear and/oranisotropic.

An especially accurately fitting dental prosthesis can be obtained, inparticular with really anisotropic or inhomogeneous blanks, if theenlargement factor is determined by a three-dimensional transferfunction f(x, y, z), the three-dimensionally transfer function f(x, y,z) expediently being determined by the three-dimensional densitydistribution F(x, y, z) of the ceramic blank.

For the largely automatic production, the method is advantageouslyfurther characterized by reading the transfer function f(x, y, z) ordensity distribution F(x, y, z) allocated to a blank to be processedfrom a data carrier, in particular if the data carrier is a bar codelabel, a transponder label which can be read out via a inputting deviceor a data bank which can be accessed via identification means attachedto or with the blank.

Further details, advantages and features of the invention can be foundnot only in the claims, in the features to be found therein—separatelyand/or in combination—but also in the following description of thepreferred embodiments found in the drawing, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic crosssection through two tooth stumps with adental prosthesis in the form of a bridge.

FIG. 2 is a flow chart showing the method steps for producing a dentalprosthesis in accordance with the claimed invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows two prepared tooth stumps 1 and 2 which are to be smoothedin their upper areas 3 and 4 for receiving a dental prosthesis. In thiscase, the dental prosthesis comprises a basic bridge structure 5 whichis, in addition, provided with a facing 6 in a conventional manner forproducing the visual and masticatory surfaces. The basic bridgestructure 5 is fastened in each case to the tooth stumps 1, 2 by meansof a layer of a dental cement in the cement gaps 7 and 8.

For reasons of clarity, in particular the gap 7, 8 such as cement gapsare illustrated completely untrue to scale.

To produce the dental prosthesis, i.e. in the embodiment, the basicbridge structure 5 with the facing 6 with the desired gaps 7, 8, amaster model of the jaw in which the dental prosthesis is to be placedis firstly produced according to the invention. The master model showsthe situation of the patient in the mouth, i.e. positively, which isobtained from an impression corresponding to the negative form. On themaster model, the dental technician, with his skilful abilities, forms amodel of the basic structure of the dental prosthesis to be produced inwax as well as plastic which melts at a low temperature and hardens in apolymerizing manner to obtain a positive model. This positive model isdigitized, whereby known optical or mechanical methods can be used. Atthe same time, a gap is generated between a later dental prosthesis andtooth stump by inputting offset values to obtain the desired fitaccuracy. The gap width is thereby axis-dependent, i.e. a distortiontakes place, in particular in the Z direction, whereas a small gapmeasurement, which can go to 0, is selected in the lower peripheralarea.

Independently thereof, the dimensioning of the basic structure or dentalprosthesis to be produced is designed such that, with normal use, amaximum tensile stress corresponds approximately to 0.1–0.7 times thestrength of the material used. To name just a few ceramics by way ofexample, Al₂O₃, zirconium oxide mixed crystals (ZrO₂/Y-TZP), MgO, Y₂O₃or TiO₂ are, for example, possible materials.

When using, for example, Al₂O₃, care should be taken that the maximumtensile stress does not exceed 245 N/mm². However, a material depositionto a maximum tensile stress of 175 N/mm² is usually sufficient.

When producing the dental prosthesis 5 with aid of the method accordingto the invention, the offset value, with which the inner surface 9 ofthe surface model is machined to form a cement gap 7, 8 between thefuture dental prosthesis 5 and tooth stump 1, 2, is either setinteractively by the operator or is definitely preset by programming.

The smallest gap dimensions in the area of the interface of dentalprosthesis 5 and tooth stump 1, 2 and thus an especially good resistanceto a later caries attack can be obtained when the gap differs from a lowvalue on the periphery 10 of the cavity defined by the inner surface 9of the positive model to a larger value in the tip 11 of the cavity, inparticular when the gap on the periphery 10 of the cavity is less than 5μm, preferably less than 2.8 μm, especially preferably not more than 1.5μm.

To avoid the formation of hollow spaces between dental prosthesis 5 andtooth stump 1, 2 from which a further caries attack of the tooth stump1, 2 and thus a destruction of the support tooth of a bridge 5 canusually be expected, it is advantageous to further conduct a test of thedata of the inner surface 9 of the positive model for undercuts withreference to the data of adjacent areas of the positive model, sinceundercuts either result in the dental prosthesis not being able to beplaced on the stump 1, 2 or a hollow space being formed.

For an especially high reliability against breakage of the dentalprosthesis 5 during later use by the patient, the wall thickness of thedata model is furthermore calculated by comparison of the data of theinner surface 9 of the positive model with the data of the outer surface12 of the positive model and comparison of the value of the wallthickness with a wall thickness minimum value which can either bedetermined by an input of the dentist or dental technician or definitelypreset by programming. When using zirconium oxide ceramic, it isexpedient if the minimum value of the wall thickness is preferably about0.1 mm to 0.3 mm.

To produce especially stable and durable bridges 5, there is preferablya calculation of the material cross-section in bridge elements in thearea 13, 14 between two posts formed by the tooth stumps 1, 2 andcomparison of the cross-section with a minimum value, which can beeither interactively set by the operator or definitely preset byprogramming, and correction of the data of the outer side 12 of thepositive model so that at least the minimum value is obtained, whichpreferably amounts to about 2 mm² to 7 mm² in yttrium oxide reinforcedzirconium oxide ceramics.

1. A method for the production of a dental prosthesis to be fastened toat least one tooth stump, comprising the steps of: producing animpression of the region of the jaw-bone comprising the tooth stump,forming a master model of the region of the jaw-bone on the basis of theimpression, forming a positive model corresponding to the dentalprosthesis to be produced, digitizing the positive model, inputtingoffset values to form a gap between the dental prosthesis to be producedand the tooth stump, calculating a program on the basis of the digitizedvalues of the positive model and the offset values for processing amaterial of a blank by a machine tool for producing the dentalprosthesis, wherein the dental prosthesis to be produced is dimensionedsuch that under consideration of the material used for the blank, innormal use of the prosthesis, a maximum tensile stress occurs whichcorresponds to about 0.1–0.7 times the tensile strength of the materialused.
 2. A method according to claim 1 characterized in that the dentalprosthesis is prepared in such a way that the maximum tensile stress inthe dental prosthesis corresponds to ≦0.5 times, in particular 0.2–0.5times the tensile strength of the material used.
 3. A method accordingto claim 1, further characterized by the following procedural steps:calculation of the wall thickness of the data model by comparing thedata of the inner surface of the positive model with the outer surfaceof the positive model and comparison of the value of the wall thicknesswith a wall thickness minimum value, correcting the data of the outerside of the positive model that at least the wall thickness minimumvalue is obtained and/or a warning is issued.
 4. A method according toclaim 3, characterized in that the wall thickness minimum value is atleast about 0.05 mm, preferably about 0.05 mm–1.0 mm, in particular 0.1mm–0.8 mm.
 5. A method according to claim 4, characterized in that thewall thickness minimum value is at least 0.3 mm.
 6. A method accordingto claim 1, further characterized by the following step: calculation ofthe material cross-section in bridge elements in the area (13, 14)between two posts formed by the tooth stumps (1, 2), comparison of thecross-section with a minimum value, correction of the data of the outerside (12) of the positive model so that at least the minimum value isobtained and/or a warning is issued.
 7. A method according to claim 6,characterized in that the minimum value of the cross-section is at leastabout 2 mm², preferably about 2 mm²–7 mm².
 8. A method according toclaim 7, characterized in that the minimum value of the cross-section isat least 7 mm².
 9. A method according to claim 1, characterized in thatthe offset value is preset dependent on the axis.
 10. A method accordingto claim 1, characterized in that the offset value is up to 150 μm. 11.A method according to claim 1, characterized in that the offset value onthe periphery of the cavity is about ≦5 μm, preferably about ≦2.8 μm, inparticular ≦1.5 μm.
 12. A method according to claim 1, furthercharacterized by the following step: conduction of a plausibility testof the data with reference to the data of adjacent areas of the positivemodel and, optionally, issuance of a warning.
 13. A method according toclaim 1, further characterized by the following step: replacement ofmissing, preferably individual, data points or non-plausible data pointsby interpolated values with reference to the data of adjacent areas ofthe positive model.
 14. A method according to claim 1, furthercharacterized by the following step: conduction of a test of the data ofthe inner surface of the positive model for undercuts with reference tothe data of adjacent areas and, optionally, issuance of a warning.
 15. Amethod according to claim 1, further characterized by the followingstep: smoothing the outer and/or surface of the data model by means of asmoothing function.
 16. A method according to claim 1, furthercharacterized by the following step: calculation of the radii of theinner surface of the surface model, comparison of the values of theradii with a minimum radius, correction of the data of the inner surfaceof the positive model so that the cavity is enlarged until at least theminimum radius is obtained and/or a warning issued.
 17. A methodaccording to claim 1, further characterized by the following step:machining the positive model with a preset offset value to adapt it tothe tool contour during the subsequent processing.
 18. A methodaccording to claim 1, further characterized by the following step:processing the data of the positive model with an enlargement factor tocompensate shrinkage or swelling of the material for the dentalprosthesis during production.
 19. A method according to claim 18,characterized in that the enlargement factor is anisotropic and/ornon-linear over the surface model.
 20. A method according to claim 19,characterized in that the enlargement factor over the surface model isdetermined by a three-dimensional transfer function f(x,y,z).
 21. Amethod according to claim 20, characterized in that thethree-dimensional transfer function f(x,y,z) is determined by thethree-dimensional density distribution F(x,y,z) of a ceramic blank. 22.A method according to claim 21, characterized by reading from a datacarrier the transfer function f(x,y,z) or density distribution F(x,y,z)allocated to a blank to be processed.
 23. A method according to claim 1,characterized in that the data carrier is a bar code label, atransponder label or a database which can be accessed via identificationmeans applied to or supplied with the blank.
 24. A method according toclaim 1, further characterized by the following step: comparison of theradii of the outer and/or inner surfaces of the positive model with aminimum radius of at least about 0.1 mm, preferably about 0.2 mm, inparticular 0.3 mm, correction of the data of the outer and/or innersurface of the positive model to the minimum radius and/or issuance of awarning.